[packagekit] Appstream dependencies

[Peter]

When appstream was added to the hif backend 
(https://github.com/hughsie/PackageKit/commit/62bc031cab55e1830a99f79ea12b7e59a9f54c3a) 
it brought in a whole bunch of gui and imaging libraries as dependencies 
when using the hif backend, which is the default on fedora. While that's 
fine for desktops, for servers we really can't install all those packages.

I don't know what the best approach for fixing this might be. Would 
having this live in a plugin be a possibility?